Hitchcraft for hammock tensioning

[blackbishop351]

Well - yes and no. The ridgeline doesn't create the extra force, the extra force is created when the ridgeline is pulled tight.

Obviously...sometimes I type too quickly for my own good. Sorry for the misuse of language there.

You're dead on with your force analysis, as far as the suspension beyond the ridgeline attachments. I'll also comment that for most purposes, the elastic forces involved can be ignored by assuming a static situation, which is perfectly reasonable. I'm not about to try and tackle a dynamic analysis

What I'm thinking about is the forces between the hammock and attachment points, as well as the tension on the ridgeline itself. I'll get on that sometime soon...maybe tonight, but now I've got beer on the way

[DGrav]

Wow! This former history and music major is much impressed! Not only by the math skills but how well you guys did explaining it to us non-math folks!

[GrizzlyAdams]

man... i didn't know yall's going to be talkin greek on here

yeah, but it's Koine Greek, so your pastor will approve.

Grizz

[slowhike]

yeah, but it's Koine Greek, so your pastor will approve.

Grizz

yeah... but it's still greek to me

[gumby]

This is one of the reasons I like this site. Not only do I learn the how's and why's, but also the background.

Thanks everyone.

[hitchman]

hitchman - I think you are overlooking the horizontal forces. So far you have considered only the vertical force, the weight, and seem to feel that the horizontal forces are not worth mentioning. But most, maybe all, here that use a structural ridgeline, pull that ridgeline very tight when hanging. So the horizontal forces are not insignificant.

I personally pulled a brand new hook of 1/4" diameter steel out into a straight line the first time I hung a hammock with a structural ridgeline. Pulled the ridgeline tight, got in the hammock, was just relaxing and got dropped to the ground. But it took a hell of a lot more than my weight to pull that 1/4" steel hook into a straight line. So I think you are not really considering all of the forces at play here.

Teedee - I did ignore the ridgeline, and I see that the discussion went on. The easiest way to estimate the load is to ignore the ridgeline and look at the angle of the cord, because the effect of the ridgeline is to reduce that angle, and the angle determines the tension in function of the weight (w/2 sin A). The same formula applies. If you go to http://hitchcraft.net/thumb_7.html there is a picture that shows an angle around 30 deg. I don't know if this is typical, but if you like to tighten the ridgeline you are increasing the load very quickly as the angle changes.

[TeeDee]

Teedee - I did ignore the ridgeline, and I see that the discussion went on. The easiest way to estimate the load is to ignore the ridgeline and look at the angle of the cord, because the effect of the ridgeline is to reduce that angle, and the angle determines the tension in function of the weight (w/2 sin A). The same formula applies. If you go to http://hitchcraft.net/thumb_7.html there is a picture that shows an angle around 30 deg. I don't know if this is typical, but if you like to tighten the ridgeline you are increasing the load very quickly as the angle changes.

Ignoring the ridgeline is by far the easiest way to go but unfortunately leads to incorrect conclusions.

In one thread someone asked the members of the forum how they hung with the structural ridgeline. If I remember correctly, every responder wrote that they tighten the suspension down as tight as they reasonably could when hanging. I know I did that also when I first got my Hennessy hammock. I can no longer remember whether I picked that up from Sgt Rock's web site or from Hennessy in some manner. Just checked the Hennessy ULBA stuff sack - it advises: "Adjust hammock until centered and ridge line is level and under light tension." Also, nearly every advisement I have read on the net about hanging the Hennessy says to pull tight, sit or lay on the hammock for a few minutes and then re-tighten. So it would seem a loose ridgeline is not used by many people.

No - the formula you have given ignores the tension of the suspension prior to any weight being added. That tension in the system and the ridgeline definitely cannot be ignored. They do 3 things:

1. The tension adds to the forces on the suspension.

2. the stretching of the ridgleine when weight is added adds more force to the suspension system.

3. the ridgeline, as most people who have described their use of the structural rideline here on the forum, limits the final suspension angle to very much less than what you are assuming or apparently use yourself judging from the picture you referenced.

1. and 2. are forces for which you have not accounted and 3. greatly increases the force applied by your formula because it limits the suspension angle from what you have assumed or use.

Now ignoring the tension applied by the user when hanging is also tempting, since there is no practical way of measuring or assuring or limiting what that force is. But it most definitely cannot be ignored.

I assume you are discussing a system with a structural ridgeline since you have stated that you personally are using a Hennessy, which I further assume still has it's structural ridgeline intact, and your video depicts a Hennessy being used. As such, you must then account for the forces which you're ignoring and which cannot be ignored and you cannot assume the 30 deg suspension angle as you have been doing.

I napped in my hammock this afternoon and again "eyeballed" the suspension angle and I am pretty sure that the final suspension angle is between 5 deg and 10 deg, closer to 10 deg. That increases the forces by a factor of approximately 3 to over 5 times what you are computing with a 30 deg angle. That is significant and doesn't account for the forces in 1. and 2.

If you are discussing solely a system without a structural ridgeline, then your formula applies as is and you are probably correct in assuming something close to a 30 deg suspension angle.

Oh by the by - you may want to change the picture you referenced and the video also - both show the hammock hung with no tree huggers. Not many forum members think that is a good thing.

[hitchman]

Ignoring the ridgeline is by far the easiest way to go but unfortunately leads to incorrect conclusions.

If you are discussing solely a system without a structural ridgeline, then your formula applies as is and you are probably correct in assuming something close to a 30 deg suspension angle.

Oh by the by - you may want to change the picture you referenced and the video also - both show the hammock hung with no tree huggers. Not many forum members think that is a good thing.

TeeDee - I'm afraid I may not have made myself clear. When I say ignore the ridgeline I don't mean that it makes no difference. What I mean is that when you analize the forces you can cut the ridgeline out of the equation. You can look at the forces by looking at the hammock as a black box suspended by two ropes. The only thing that you need to know to calculate the forces on the rope (and rope hardware) is the angle and the load. SO with or without ridgeline, what matters is the angle of the rope when you are hanging on the hammock.

I did use a 30 degree angle, and I don't dispute that as the angle gets smaller the rope tension increases very fast. In fact I offered that on my last post. Now from many pictures and my own use I don't know that there is an advantage in tightening the hammock so much that the angle is 10 degrees. That puts huge loads on the hammock components.
I don't want to get too technical but to take this argument to conclusion I have to. By reverse engineering my HH I find that the rope has a breaking load of 1600 lbs. The manufacturer's suggested working load on a rope is 20% of that limit (from Samson Ropes literature). That tells me my hammock was designed for a maximum load of 320 lbs on the rope, which at 30 degrees means a 320 lb weight. That results in a safety factor of 1.28 for the design load of 250lbs. If you reduce the angle the factor of safety goes away. You might want to check with the manufacturer what their recommendation is, but as a product developer this is how I make my assessment.

Regarding the tree huggers, I'm with you. The picture was taken when I filmed a short instructional video about the hitchcraft tie and it was a way to keep it simple. In fact, the hitchcraft ties work better with tree huggers as long as there is enough distance between the trees.

[Coffee]

hitchman, I may have missed this in an earlier post, but how many hangs do you have on the same set? Actually how many short hangs or over nights.

[hitchman]

Are you referring to the life of the product?
I don't have much data, but I have a demo that has been used for 100-150 cycles and the teeth start to show some wear marks. I would estimate that they still have at least 70-80% of their life left and would have to be replaced after 500-1000 cycles of hard use.
I would expect that they would last for the life of the hammock and still be fully functional when the hammock components (seams, ropes) start to fall apart.